Plácido Domingo's name means Placid Sunday, which is not what he will be enjoying today. He may well sleep until the afternoon, but will make up for that inertia tonight on stage at the Royal Albert Hall, in a BBC Proms performance of the Royal Opera's production of Verdi's Simon Boccanegra, he will age half a century, beginning as a swashbuckling corsair and ending – after the lapse of a few decades between the acts – as the elderly, careworn Doge of Genoa, poisoned by a vindictive political crony.

Since Domingo never disappoints, there will be the usual expenditure of vocal energy, in a graver, mellower register than we heard in his heyday as a virile tenor, when he wailed in erotic torment as Don José in Carmen or sounded a revolutionary battle cry as the freedom-fighter Cavaradossi in Tosca. Now 69, Domingo has relaunched himself as a baritone; although Boccanegra spends the second half of the opera melodiously dying, what we will be witnessing is the reincarnation of a performer who is, despite his white hair, ageless and indomitable. There is extra cause for jubilation, since Domingo has recovered from surgery earlier this year to remove some cancerous polyps from his colon. The emergency caused him to cancel his performances in Handel's Tamerlano at Covent Garden, but within a month he was commuting between the opera companies he runs in Washington and Los Angeles while singing throughout Europe and taking a side trip to open an offshoot of his Manhattan restaurant Pampano on a man-made island in the Persian Gulf. It will of course end with stamping, whistling, cheering pandemonium. Tomorrow Domingo will be back in the air, where he can usually be found if he is not on stage.

"After Boccanegra," he told me five years ago when first planning this baritonal epilogue to his career, "I will probably say Amen." Mindful of his surname, he crossed himself to signal his seriousness. But as the promised end approaches, Domingo seems intent on postponing it. This September he will sing another of Verdi's baritonal patriarchs, the tragic jester Rigoletto, in a performance to be relayed on television from Mantua; in 2012 there will be a third baritone role, the lapsed monk Athanaël in Massenet's Thaïs, to commemorate the centenary of the composer's death.

Domingo has always been admired or adored, but recently he has undergone a kind of deification. In Mexico City, where he grew up, they have turned the man into a monument: a statue of Domingo, arms outspread in an operatic gesture that embraces the world, repays him for his fundraising efforts after the 1985 earthquake, in which some of his relatives were killed. He deserves such tributes, but there is a profounder, truer explanation for the wonder he excites. Great performers, whether on a football pitch, a tennis court or in a theatre, live by their wits, keep going by the grace of their fragile bodies, and while practising their skill seem to exist in a perpetual present, stopping the clock in moments that go on for ever. Hence those slow-motion replays of improbably angled goals or zigzagging volleys, and hence too our sense that a sonorous B flat or high C has lasted for a lung-busting eternity rather than a few seconds. These rare creatures are exempt from the physical limitations that drag us all down; they are figures of athletic and aesthetic transcendence. For that very reason, they generally have a short lease of life. But Domingo – who conscientiously keeps score of the records he has broken, and can tell you that he has sung 225 performances of Verdi's Otello (including three as the other tenor, the philandering Cassio) and 224 of Tosca (including two as the police informer Spoletta) – is like a Federer who goes on winning Wimbledon for three decades or a Rooney who doesn't use his inflated reputation as an excuse for underachieving.

His website proclaims his catchy creed: "If I rest, I rust." Put another way, that means: "If I stand still, I die." There is a thrilling existential frenzy to Domingo, despite his unoperatically low-key personality. He has defeated time by conserving his gift for so long, despite tearing passions to tatters in every performance, and has done his best to defeat space by living simultaneously in divergent time zones. Days off are anathema, and he used to persuade opera houses to synchronise schedules so that he could, for instance, sing one opera in New York while conducting another in Chicago on the days in between. Three weeks ago, after a late-morning dress rehearsal of Boccanegra at Covent Garden, he flew to Vienna for a gala that evening; he returned to Vienna the night after Boccanegra opened to sing the last 15 minutes of Wagner's Parsifal to mark the end of the administrator Ioan Hollander's regime. Last weekend his earthbound feet began itching again, so he flew to South Africa to watch his national team win the World Cup, nipping back to London in time for another Boccanegra on Tuesday.

Performers who last long enough lead us through the successive ages of man, which is why Domingo's accelerated ageing in Boccanegra is so moving. "I must show to those who are younger," he said to me in 2005, "how live theatre is supposed to be." Live theatre means living dangerously, with a calorific intensity that would exhaust most of us; but that very fury accelerates the tempo of existence and anticipates death. Domingo's dusky, elegiac timbre made him is a natural tragedian, and he usually expired – impaled on his own sword as Otello, gunned down by a firing squad as Cavaradossi, guillotined as Giordano's Andrea Chénier, stabbed during a mazurka in Verdi's Un Ballo in Maschera – at the end of the evening. If he survived, it was to sob over a lost love like Des Grieux in Puccini's Manon Lescaut and Rodolfo in La bohème, or to mirthlessly mock his own downfall like the murderous clown Canio in Leoncavallo's Pagliacci. Since he dislikes wasting time, my interview with him took place backstage at Covent Garden during the second act of Die Walküre, while Wagner's other characters were lengthily debating the fate of the young warrior Siegmund, played by Domingo. He sat talking to me while listening with one ear to a relay from the stage; when the time came, he picked up his sword, had some fresh wounds painted on to his battle-scarred body, then strode off to die once again, as if for the first time.

He is ending late, and – if you accredit the official chronology, which gives his birthdate as 1941 – he began very early. He was married at the age of 16, had his first son immediately, then promptly got divorced; still only 17, he sang Freddie in My Fair Lady in Spanish in Mexico, and by the age of 20 was appearing with Joan Sutherland in Donizetti's Lucia di Lammermoor in Dallas. Some reference books make it all seem less breathlessly precocious by testifying that he was born in 1934, though when I mentioned this hypothesis in a review a while ago, I received a politely pained letter from his New York publicist, who enclosed smudged photocopies of his client's birth certificate and, for good measure, a document attesting to his baptism. I'd advise Domingo to add a few years rather than deducting them: it makes his defiant vigour all the more precious.

Whatever the truth about the dates, he has lasted so long because of a strict emotional economy. Pavarotti was as uncontrollably extroverted as a puppy that is not yet housebroken. Domingo, by contrast, has a dignified reserve that is deeply Spanish. The only tantrum of his that I know about was an impromptu walkout from the dress rehearsal of Verdi's Don Carlos at the Met in 1983, provoked by his discovery that the New York Times photographer had bypassed him in favour of two Afghan hounds led about by the soprano Mirella Freni. Otherwise he is equable, friendly to all comers, and thus able to fend off unwanted intimacy. Once in San Francisco, after a performance of Samson et Dalila, I watched him cope with a scantily clad socialite who offered all her available limbs and begged "Please sign me!" He chose her wrist as the safest option, and scrawled his name with her felt pen. His endearing propriety dictated his answer when I asked him whether he disliked any of the characters he had played. He nominated Rasputin in Deborah Drattell's Nicholas and Alexandra, commissioned by his company in LA. "He is a drunkard, a man who is filthy in his habits, and he uses his connections with power to have all these women – bah!"

"I keep everything for the stage," he told me, "except happiness. That I want to have when I am not in the theatre – when I am with my family." The family consists of his second wife, Marta, an owlishly bespectacled and beaky Mexican soprano who now directs operas for the companies Domingo runs, and their two sons. The clan foregathers in Acapulco at Christmas, but I imagine that Domingo spends the holidays waiting to resume the revved-up life and slowed-down death of an operatic hero. "The stage is the place for violence and anguish and big tragedies," he said. "It is wonderful if we can confine the disasters in this way. I am a happy man, but I love to suffer on stage, it is the most beautiful thing of all!" Whether he knew it or not, he had paraphrased Aristotle's doctrine of tragic catharsis: we are purged and exalted by watching someone else's mental distress and physical torment. Domingo, so unforgettable in the tortured laments of Puccini's characters, is our designated sufferer.

My own best memories of him go back to the 1980s, which dates us both. He was extraordinary in Otello because he is himself so unvolcanic; it was terrible to watch this confident, affectionate man lose his bearings, to see him stumbling and staggering in bewilderment, with the untarnished bronze of the voice showing off his earlier nobility. As the poetic wastrel in Offenbach's The Tales of Hoffmann he became another man who could not have been more unlike him – self-indulgent and self-destructive, squandering his genius rather than guarding it as Domingo has done. In Don Carlos, once he recovered from his tizz about the Afghans, he got inside the skin of a man who did not understand the source of his Oedipal aggressions and seemed – like the historical character, the neurotic son of Philip II of Spain – to be physically deformed by despair, hunched and limping.

Pavarotti's inimitable sound was like tuned, effervescent laughter. What you heard pouring from Domingo's taut throat in his younger days was raw, wrenching anguish (which is why his sometimes strained high notes had such affecting power, as if he were hurt into eloquence). This has now, in Simon Boccanegra, softened and deepened into regret, remorse, a longing – as he first regains the daughter who was taken from him, then loses her to a lover who is his political enemy – to reverse history, expunge our guilt, and begin all over again. He is, after all, more than a vocal athlete. Like a priest, he holds out the possibility of happiness when he sings with tender, caressing pianissimi. Like a healer, he helps us to come to terms with the miseries he ventilates on our behalf. In Boccanegra he even leads an ensemble that calls for peace in the world and brings it about in a magical trill, voiced by the soprano, that hovers in the air like an angel. I first heard Domingo in Un Ballo in Maschera in New York in 1972, and realise I have taken him for granted for almost 40 years. With a kind of panic, I am starting to wonder whether I'll be able to cope without him.